This is a continuous effort to investigate the relationship between the onset and progression of spontaneous reticulum cell sarcomas of SJL/J mice and the host anti-tumor immune competence. In this application, we propose to investigate the following; (1) To demonstrate wether the mixed leukocyte tumor reaction (MLTI), obtained readily in vitro as assessed by 3HTdR incorporation, can also be generated in vivo. (2) To characterize the nature of cell-cell interactions which mediate an MLTI reaction and delineate the nature of cells which interact with tumor cells at various stages of neoplastic growth. (3) To characterize the molecular nature of RCS antigens which trigger a proliferative response. (4) To investigate the mechanisms by which RCS tumor cells fail to stimulate a cytotoxic T cell response. (5) To evaluate the role of antibody mediated enhancement as a possible mechanism in tumorogenesis.